Download Shepherd Moons

Satellite A
Shepherd Moons
Particle
Consider a moon (it can be tiny) orbiting just
outside a ring. Ring particles, being closer to
the planet than the satellite, are moving faster.
If a particle drifts out of the ring as the first
figure, when it overtakes the satellite, the
satellite's gravity will cause the particle to lose
kinetic energy and drag the particle back, slow
it down, and cause it to fall back into the ring.
Ring
Similarly, a moon can orbit just inside a ring as
the second figure. The satellite is moving faster
than the ring particles. If a particle drifts inward
from the ring, it will be overtaken by the
satellite. The satellite's gravity will speed the
particle up and throw it back into the ring.
Ring
Particle
Satellite B
Saturn has a thin ring (the F ring) bounded by
two tiny moons that keep it confined. Some of
the shepherd moons for the thin rings of
Uranus and Neptune are also known.
Source of diagrams and text: Steven Dutch, Natural and Applied Sciences,
University of Wisconsin - Green Bay
https://www.uwgb.edu/dutchs/planets/resonanc.htm